Stereotaxy refers to a technique, most often applied to the nervous system, in which the contents of a patient's skull (or other portion of a patient's body) are considered in a precise three-dimensional space defined by a measuring instrument. Traditionally, the measuring instrument used to perform stereotaxy is a stereotactic frame, typically taking the form of a cage structure, that is fixed to the patient's skull or other part of the patient's body. Stereotactic frames are mechanical devices typically based upon a Cartesian or polar coordinate system. These systems typically include a means for securing the stereotactic frame device to the patient, at least one measuring scale for determining and confirming target coordinates and probe trajectories, and a probe holder or carrier.
The probe holder or carrier is used to direct a surgical probe or some other instrument to a desired three-dimensional location within the work space that is defined with respect to the geometry of the stereotactic frame. In a typical application, the stereotactic frame is used to position a probe or other instrument inside the body into an anatomic or pathological structure. The frame coordinates for the target structure are determined from stereotactic imaging studies performed using computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography, etc. For CT and MRI based stereotaxis, the coordinates for an intra-cranial target are derived from stereotactic CT and MRI imaging examinations that are performed with the patient's head fixed in the rigid, confining, stereotactic frame. CT and MRI opaque external fiducial reference marker systems are applied to the frame to facilitate and simplify the calculation of stereotactic coordinates from the imaging.
Computer based interactive stereotactic methods allow tumors identified by CT and MRI to be considered as volumes in space and provide a surgeon with graphical displays that indicate the CT and MRI defined boundaries of the lesion within a defined stereotactic surgical field. In these procedures a tumor volume is reconstructed from stereotactic CT or MRI data and reformatted along a surgical viewing trajectory defined by a stereotactic frame. During surgery, an operating room computer system displays cross sections of the reformatted tumor volume with respect to surgical instruments directed into the surgical field using the stereotactic frame as a reference source. Intra-operatively, the surgeon monitors the computer-generated image of the surgical field which was derived from CT or MRI scans, as well as the surgical field itself. Systems have been developed for the super-imposition of the computer image upon the surgical field by means of a heads-up display unit attached to an operating microscope. This allows the surgeon simultaneously to view updated reformatted and scaled images of the CT or MRI defined surgical field visually superimposed upon the actual surgical field.
Stereotactic systems using stereotactic frames have a series of limitations. Stereotactic frames are cumbersome in general. They are especially cumbersome for procedures requiring more than a few target points and in volumetric stereotactic procedures where the demands of the procedure dictate the need for a larger working area, yet where such demands come into conflict with the physical structure of the frame. A conventional stereotactic frame is typically a cage structure extending about the patient's head that inherently restricts freedom of movement of the surgeon. Changing a target point or trajectory to reach a target point involves a mechanical adjustment of the stereotactic frame. Many such mechanical adjustments become very cumbersome when a surgeon is confronted with an infinite number of points from which to define a boundary of a volumetric lesion. In addition, the stereotactic reference frame must be applied to the patient's head in order to acquire pre-operative images. Some surgeons find the stereotactic frame application procedure difficult and time consuming. Patients also find this uncomfortable. In addition, the necessity to repeat CT and MRI examinations with the frame attached for obtaining pre-operative images increases the cost to patients.
In order to overcome the limitations associated with using stereotactic frames, various “frameless” stereotaxy systems have been developed. Frameless stereotaxy systems enable stereotactic guided surgery with a minimal structure present, thereby minimizing interference with the surgical procedure while providing enhanced accuracy in stereotactic imaging assistance to the surgeon. In a frameless stereotaxy system a probe or other medical instrument or device in the hand of a surgeon is tracked in three-dimensional space using an optical, electromagnetic, or other tracking system that interferes only minimally with the surgeon's use of the instrument. Pre-operative MRI or CT images are registered to the surgical field by indicating, with the stereotaxy system probe, locations on the patient's body or otherwise within the surgical field that correspond to points in the pre-operative images. For example, markers may be placed on a patient's anatomy (e.g., on the scalp or skull) while the pre-operative MRI or CT images are obtained. The stereotaxy system probe is then used to indicate the same marker positions in the real three-dimensional space under observation of the stereotaxy system after the patient has been positioned for surgery. Once the frameless stereotaxy system is registered in this manner, the surgeon may use a graphical display of the pre-operative images (e.g., rendered into a three-dimensional graphical representation) as registered to the surgical field, as provided by the system, accurately to position the probe, or other instrument, whose position is being tracked by the system, into a desired position in the patient's body. The frameless stereotaxy system display shows the tracked position of the probe or other instrument in real space relative to the registered pre-operative imagery
Brain-shift is a major limitation of all neuro-navigational systems that rely on pre-operative imaging data. With ongoing surgery a remarkable deformation of the brain tissue may occur, resulting in navigational localizing errors as the real position of brain structures varies from that indicated in the pre-operative imaging. Brain-shift, also known as post imaging brain distortion or brain deformation, summarizes the behavior of brain tissue during surgical manipulations, i.e., opening of the cranium and dura, resection of tissue, use of brain retractors, and loss of cerebral spinal fluid. Negative or positive brain-shift, corresponding to infalling of the brain or swelling or expansion of compressed normal brain tissue because of tumor debulking, may occur. Experienced neurosurgeons can use various techniques to minimize the effects of brain-shift during the surgical procedure.
The shifting of cortical structures must be taken into account when superficial tumors adjacent to eloquent cortical brain areas, such as the motor area or speech related areas, are removed. This shifting often does not concern neurosurgeons, because in most cases the shifting is clearly visible during the surgical procedure and therefore can be easily compensated for. However, shifting of deep structures, so-called subsurface shifting, is much more relevant. Shifts of a deep tumor margin can lead to incomplete or too-deep resection. Subsurface deformation must be taken into account when neuro-navigation systems, which cannot be trusted for evaluation of deep tumor margins once surgery has begun, are used.
As tumor resection proceeds, brain-shift may result in significant inaccuracy of the navigation system for a high percentage of patients, so that as the operation proceeds some correction of the navigation system must be performed if it is to be relied upon. Intra-operative imaging using MRI provides the possibility of updating the navigation system with real data on the deformed brain. Intra-operative updating depends on the availability of an intra-operative MRI system. It is not only cost intensive but also time consuming. Intra-operative ultrasonography may be a choice for real-time updating of neural-navigation systems in the future. Ultrasonagraphic data could, for example, be used automatically to deform a preoperative MRI scan to match the shifted brain.
Different mathematical models that attempt to describe the brain-shift phenomenon have been developed in recent years. Some try to compensate for the brain-shift by using an intra-operatively guided deformable model that is based on the modeling of brain tissue as a homogenous linear viscoelastic material. Finite-element models describing the deformation of brain tissue are under investigation. However, there are inter-individual differences in brain-shift behavior, and brain tissue properties might change during surgical procedures, because of changes in intra-cranial pressure or blood carbon-dioxide concentrations or because of alterations of the water content of the brain after the administration of osmotically active drugs.
Attempts have also been made to modify preoperative imaging data, so that the images match the real intra-operative deformed brain, by measuring only several points during the surgical procedure, e.g., using optical scanning of the brain surface or ultrasonography. These limited intra-operative data are called sparse data.
A much more simple method that has evolved for mitigating the problems associated with brain-shift during tumor removal involves placing flexible catheters or pledgets around the tumor boundary using stereotactic frame navigation early in the procedure, before resection. The catheters provide visible limits to the resection that will track any shift or deformation that occurs. Thus, intra-operative re-imaging is not required.
What is desired is an improved system and method for using frameless stereotaxy to mark the margins of a brain lesion or other internal structure such that the ability of a neurosurgeon completely to remove a tumor while minimizing the removal of surrounding normal tissue is improved despite brain-shift that may occur during the resection process.